How to populate a List which has a class within a class

JSON:

{
    "Col1": "John",
    "Col2": "Doe",
    "Col3Combined": [
        {
            "Col3": "12",
            "Col4Combined": [
                {
                    "Col4": "156-345"
                }
            ],
            "Col5Combined": [
                {
                    "Col5": "792-098"
                }
            ]
        }
    ]
},
{
    "Col1": "Mike",
    "Col2": "Keller",
    "Col3Combined": [
        {
            "Col3": "12",
            "Col4Combined": [
                {
                    "Col4": "145-394"
                }
            ],
            "Col5Combined": [
                {
                    "Col5": "156-323"
                }
            ]
        },
        {
            "Col3": "15",
            "Col4Combined": [
                {
                    "Col4": "909-203"
                }
            ],
            "Col5Combined": [
                {
                    "Col5": "121-444",
                    "Col5": "134-232"
                }
            ]
        }
    ]
}

C# classes:

public class Col4Combined
{
    public string Col4 { get; set; }
}

public class Col5Combined
{
    public string Col5 { get; set; }
}

public class Col3Combined
{
    public string Col3 { get; set; }
    public List<Col4Combined> Col4Combined { get; set; }
    public List<Col5Combined> Col5Combined { get; set; }
}

public class RootObject
{
    public string Col1 { get; set; }
    public string Col2 { get; set; }
    public List<Col3Combined> Col3Combined { get; set; }
}

I am doing the following:

List<RootObject> lr = new List<RootObject>();
lr.Add(new RootObject
{
    Col1 = "test",
    Col2 = "test2",
    Col3Combined = new List<Col3Combined>(); //but then how do I populate it?
}
);

How can I populate Col3Combined in the list above.